I see a lot of people asking for Black and White (Grayscale) icons, so I thought it might be worthwhile to illustrate why this might not be a high priority worth implementing.
As of 2015, Most android devices have an LCD screen (not AMOLED). B/W icons are unlikely help an LCD save energy at all. There have been tests that even showed some LCD screens actually use slightly less energy with a completely white screen (not black), this is because all light energy from an LCD screen is generated by the backlight (which is white light). An LCD screen creates black pixels on the screen by closing the pixel and blocking light from passing through. So a black (or colored) pixel on an LCD screen actually does nothing to reduce the amount of energy used.
As for AMOLED screens, yes they will save energy by having pixels display as black, because every pixel on an AMOLED screen generates its own light. An AMOLED Black pixel is "off" and uses no energy. Black pixels are good, so what about Grayscale? Gray pixels emit a Limited amount of light, but the same statement is true for Colored pixels. Grayscale icons have been advertised as battery efficient, but they may not have as a great effect of power saving as we have been lead to believe.
Linked below is a 1.5 hour "Screen On" test I did to measure the power savings of using Color vs Grayscale screen. The difference in battery use was 1%. With a measured difference that low, it is possible that the difference could be random variability, and not actually any power savings at all.
For those reasons, it may not be very beneficial to display Grayscale icons. The user is likely to experience much greater power savings from simply turning down the brightness on their screen.
Thanks for the App. It looks like it's coming along very good. To give it a much stronger ability to save power, I look forward to it eventually using root permissions to shutdown system processes, similar to Samsungs UPSM.